The registration of image upon image is important in printing, especially when making color prints. Wall printing is done in a single print engine, macroscopic registration techniques suffice. For print engines that use roll or web fed paper sources, the roll or web is generally clamped by the machine and macroscopic registration, i.e. the registration of one image upon another over the entire print receiver, is generally accomplished. For example, cyan, magenta, yellow, and black color separations can be sufficiently accurately registered by tracking the entire receiver.
In a sheet fed printing engine, registration is often more problematic than in a roll fed print engine. In a sheet fed print engine, each sheet of paper moves from one module that prints a specific color to the next, which prints another color. Each color must be kept in registration with each other color. This is generally accomplished using macroregistration whereby either the position of the sheet of paper is tracked by locating one or more edges of the paper or fiducials are printed on the page for each color and the timing and/or lateral positioning of the image printing made on modules within a print engine is adjusted to register the images. Conventionally such approaches make adjustments to the printing process that are applied uniformly such as magnification variations.
In digital printing, especially in digital printing requiring more than one type of printing or more than a single print engine to print the image, it is not sufficient to simply macroscopically register images. Rather, the heating associated with fusing in an electrophotographic printing process shrinks localized portions of the paper as moisture is emitted from the paper. Reabsorption of moisture can result in subsequent swelling of the paper. The degree of shrinking and swelling can vary from sheet to sheet and from one site on the paper to another on a sheet and can be random and non-uniform.
In particular it will be understood that many liquid absorbent and semi-absorbent receivers used in printing are dried to a moisture content of approximately 5% by weight, corresponding to the moisture content of paper equilibrated at room temperature to a relative humidity of approximately 40 to 50%. The drying of paper during production creates generally flat sheets however during such drying stresses are induced in the paper. During ink jet printing however, a substantial volume of fluid is rapidly reintroduced into the paper and this can have the effect of non-uniformly releasing the balance of stresses that maintain the flatness of the dry paper. This causes bending and warping of the paper causing localized spatial distortions not only in the plane of the paper but also in a direction that is perpendicular to the paper. This makes the likelihood of image defects greater as the paper is not at the distance that an inkjet print head expects the paper to be at during printing and also increases the surface area of the receiver in the vicinity of the distortion which then results in a distorted image.
Moreover, the swelling that occurs upon absorption of moisture generally does not occur in the locations or have the correct size to correct for the shrinkage. The magnitude of these distortions is not predictable. As a result, misregistration on the pixel level between prints can occur. The absorption of fluid especially water from a hydrophilic ink can cause the paper to locally swell. Subsequent drying does not have the effect of restoring either the shape or the original size of the paper, creating distortions which might not correspond in either location or magnitude to the previous swelling. This can cause misregistration of images on a microscopic scale even if macroscopic registration is maintained. This is consistent with the common experience of the effects from wetting and drying a flat sheet of paper.
Accordingly, what is needed is a method to correct for such microscopic misregistration. Specifically, the distortions can result in the positions of pixels, letters, characters, or other image specific data shifting despite the fact that the receiver may be macroscopically in register. The shift in the location of this information can result in the misregistration of certain specific pixels despite the fact that, overall, the images are in register when multiple printers are used. This can be especially problematic in electrophotographic technology which has a drying effect on a receiver used in conjunction with inkjet printing which as noted above provides a drying effect.